The invention is based on a priority application EP 01 440 185.5 which is hereby incorporated by reference. The invention concerns a method for the amplification of WDM signals in a WDM transmission system, wherein pump light is injected into an optical waveguide of the optical transmission system contrary to the transmission direction of the WDM signals.
Nowadays, modern optical transmission systems or networks primarily use the so-called wavelength division multiplex (WDM) method, in which a number of modulated optical carrier signals, whose frequencies differ from one another, are simultaneously transmitted in the optical waveguide. The mutual optical interference (crosstalk) of the individual modulated carriers is so small in this case that each of the said carriers can be considered as an independent wavelength channel or WDM channel. In modern WDM (transmission) systems with so-called dense wavelength division multiplex, a multiplicity of channels, for example 80 channels, are realized with a dense frequency pattern, for example, with an equidistant channel spacing of, for example, 50 GHz.
A problem of WDM transmission is the frequency-dependent attenuation of the transmitted light, as a result of which different WDM signals undergo differing attenuation. Likewise, optical amplifiers, for example erbium-doped fiber amplifiers, which are frequently used as intermediate amplifiers on a long transmission link, have a so-called gain spectrum; i.e., the WDM signals undergo a gain that is dependent on their respective frequencies. This transmission spectrum is dependent on, amongst other things, the temperature. Finally, differently progressing aging processes of the optical or opto-electrical elements of the WDM transmission system, for example, of laser diodes, or defects of optical connections, can result in significant intensity losses of individual WDM signals.
Various solution approaches are known from the prior art which are aimed at preventing drifting apart of the intensities of the WDM signals or at compensating intensity differences. Thus, the patent specification U.S. Pat. No. 6,049,414, entitled xe2x80x9cTemperature-compensated rare earth doped optical waveguide amplifiersxe2x80x9d, discloses a system and a method for compensating the influence of temperature on the gain spectrum of a fiber amplifier by means of controllable, temperature-dependent optical filters. In the case of a further method, disclosed in the patent specification U.S. Pat. No. 6,151,160, the WDM signals as a whole first undergo broadband amplification and are subsequently distributed, in dependence on frequency, to different optical branches where each of the corresponding subsets of the WDM signals are individually amplified and subsequently undergo optical recombination.
However, the known methods for intensity equalization of WDM signals have various disadvantages. The disadvantage of compensation by means of controllable filters is that it may be necessary to accept high attenuation losses. Thus, for the purpose of equalizing the intensities, the intensities of the channels, other than the WDM channel with the least intensity, are lowered to the intensity of the latter.
The disadvantage of compensation by means of distribution to different optical branches consists in, firstly, a large circuitry requirement and, secondly, intensity losses due to the splitting of the light. The known methods are particularly unsuitable when individual WDM signals have a significantly lesser intensity compared with other channels due, for example, to aging of respective components.
The object of the invention is to create a method and equipment for performing the method, in particular, an optical amplifier, in which WDM signals selected from the totality of the transmitted WDM signals can be individually amplified without influencing of the other WDM signals.
The known methods for intensity equalization of WDM signals do not permit single or individual amplification of selected WDM signals. The fundamental concept of the invention consists in making technical use of a physical effect, by means of a so-called stimulated Brillouin scattering, which permits specific amplification of individual WDM signals.
The stimulated Brillouin scattering SBS is a sound wave scattering process which occurs when coherent electromagnetic waves, a so-called pump light, of high intensity is directed into an optical waveguide. The Brillouin scattering is a non-linear effect which increases superproportionally as the intensity increases. Due to interaction with the optical medium, acoustic waves are produced which result in a diffraction grating which moves at a certain speed in the same direction as the pump light in the waveguide. This pump light is consequently scattered, the greatest scatter portion being formed by a portion of the pump light which is scattered back in the direction opposite to that of the pump light. Due to the Doppler effect, the frequency of this backscattered wave is reduced by the sound frequency of the acoustic wave. This so-called Brillouin frequency is a characteristic quantity of the respective fiber material.
If a signal of the same wavelength as the backscattered pump light is then injected into the waveguide in the direction opposite to that of the pump light, the above-mentioned acoustic diffraction grating is substantially amplified. This results in an increase in the intensity of the backscattered pump light and, consequently, in amplification of the injected signal. These interrelationships are known from the prior art.
This effect, by which pump-light energy is transferred into a signal light, is now utilized according to the invention for the purpose of amplifying selected WDM signals, in that pump light with a given frequency spectrum is injected into the optical waveguide contrary to the transmission direction of the WDM signals. The frequency spectrum of a pump light for the distortionless amplification of a selected WDM signal is in this case offset, relative to the frequency spectrum of this WDM signal, by the above-mentioned Brillouin frequency. Since the Brillouin scattering is a narrowband effect, selected WDM signals can be individually amplified using the described method.
The advantage of the invention is that amplification of individual WDM signals can be performed selectively, while practically precluding influencing of other WDM channels. A further advantage is that the gain control is performed on the receiving side, i.e., a receiving device, for example, for processing or converting the WDM signals, can accurately adjust the intensity of one or more WDM signals such that the error rate on reading out of the corresponding data is minimized.
Further developments of the invention are disclosed by the dependent claims and the following description.